U.S. patent application number 11/787258 was filed with the patent office on 2008-10-16 for multiplexing spectrometer.
This patent application is currently assigned to ASE Optics, Inc.. Invention is credited to Todd Blalock, Christopher Cotton.
Application Number | 20080252885 11/787258 |
Document ID | / |
Family ID | 39853421 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080252885 |
Kind Code |
A1 |
Blalock; Todd ; et
al. |
October 16, 2008 |
Multiplexing spectrometer
Abstract
A multiplexing spectrometer measures at least one parameter,
such as temperature, pressure or stress. The system multiplexes the
outputs of Bragg stack sensors deposited at the distant ends of
optical fibers brought in contact or in close proximity to objects.
The spectrometer detects the peaks of the optical signals returned
from the Bragg stacks and converts them into corresponding values
of the parameters of interest. The spectrometer includes an optical
system that comprises an entrance slit, a diffraction grating as a
light dispersing means. Multiplexing occurs on a two-dimensional
solid state matrix photo detector detects and converts the light
signals returned from the Bragg stack sensing elements into
corresponding electrical signals, and a built-in look-up table to
provides the values of the parameters of interest that correspond
the spectral characteristics of the returned light signals.
Inventors: |
Blalock; Todd; (Penfield,
NY) ; Cotton; Christopher; (Honeoye Falls,
NY) |
Correspondence
Address: |
JOHN M. HAMMOND;PATENT INNOVATIONS LLC
150 LUCIUS GORDON DRIVE, SUITE 205
WEST HENRIETTA
NY
14586
US
|
Assignee: |
ASE Optics, Inc.
|
Family ID: |
39853421 |
Appl. No.: |
11/787258 |
Filed: |
April 16, 2007 |
Current U.S.
Class: |
356/328 ;
356/326; 374/E1.005; 374/E11.016 |
Current CPC
Class: |
G01K 1/026 20130101;
G01M 11/083 20130101; G01D 5/35316 20130101; G01K 11/3206
20130101 |
Class at
Publication: |
356/328 ;
356/326 |
International
Class: |
G01J 3/28 20060101
G01J003/28 |
Claims
1. A system for spectral analysis of parameters comprising sensing
elements responsive to said parameters, optical fibers conducting
light from a light source to and returning from said elements to
provide a plurality of optical signals, a multiplexing imaging
spectrometer including optics for multiplexing said optical signals
by imaging said signals onto a two-dimensional photo detector
array, and means responsive to the magnitude of said signals on
predetermined locations on said photo detector array for measuring
said parameters.
2. A system in accordance with claim 1 in which said sensing
elements are Bragg stacks deposited on the distal ends of said
optical fibers and are either in direct contact with or are in
close proximity of the object.
3. A system in accordance with claim 2 in which all said sensors
are substantially identical in their dimensions and operating
parameters for interchangeability thereof.
4. A system in accordance with claim 2 in which said optical fibers
are fused at the input to said spectrometer to form an entrance
slit to said spectrometer.
5. A system in accordance with claim 1 in which said spectrometer
includes a spatially and spectrally dispersing means.
6. A system in accordance with claim 5 in which said spatially and
spectrally dispersive means are selected from a reflecting
diffraction grating, a transmission diffraction grating, and an
optical prism
7. The system of claim 1 wherein said light source generates output
radiation of sufficient magnitude to enable said sensing system to
perform the desired measurements.
8. A system per claim 1 in which said light source one of: a light
emitting diode, a super light emitting diode, a solid state diode
laser, an incandescent lamp, and an ionized gas light source
9. A sensing system in accordance with claim 5 comprising a power
supply for said light source which compensates for undesirable
fluctuations of light output.
10. A sensing system in accordance with claim 1 in which said photo
detector array is composed of two-dimensional matrix of solid state
photo detector elements that accept the light signals and converts
them into corresponding electrical signals.
11. A system per claim 10 wherein said rows and columns of said
photo detector elements accepts said light signals from only one of
said sensors.
12. A photo detector array in accordance with claim 11 in which the
location of said photo detector elements corresponds to the
wavelength of said light signals that have the maximum energy
within the particular segment of the spectrum imaged on said row of
the photo detector elements.
13. A sensing system in accordance with claim 12 in which said
elements of the solid state photo detector are selected from:
charge-coupled devices, charge injection devices, complimentary
metal oxide semiconductor devices, and avalanche diodes
devices.
14. A sensing system in accordance with claim 1 in which said
spectrometer is operative to detect the peak wavelength and the
peak magnitude of said light signals.
15. A sensing system in accordance with claim 1 in which said
spectrometer includes a monochromator such as but not limited to: a
monochromator incorporating a diffraction grating. an optical prism
monochromator
16. A sensing system in accordance with claim 1 in which a
electronic look-up table is included as a reference for converting
said peak the wavelength and magnitude of said light signals into
the corresponding values of parameters of interest.
17. A system in accordance with claim 2 in which all said sensors
are equipped with said Bragg stacks that have substantially
identical structures resulting in substantially identical responses
to variations in the magnitude and position of the wavelength peak
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method and apparatus for
optically sensing of physical parameters and particularly to a
system having a plurality of multiplexed fiber optic sensors, which
generate optical signals in response to changes in temperature,
pressure or stress in objects being monitored.
BACKGROUND OF THE INVENTION
[0002] Many types of sensors acquire and transmit information using
light signals in which the intensity of light and its spectral
signature represent the magnitude and nature of the parameter being
measured, the former by the maximum height of the peak of the
energy in the spectrum, the latter by the location of such peak.
One convenient way to process such information from a single sensor
and to convert it into its electrical equivalent is to use a
spectrometer.
[0003] In many applications, however, a network of sensors is used
to collect information from multiple locations on the objects or in
the environment being monitored. While it is also possible to use
currently available spectrometers as read-out instruments to
interrogate multiple sensors, such instruments can be complex,
expensive and not capable of operating at an adequate speed.
[0004] The class of photonic sensors that increasingly find diverse
applications for measuring and monitoring a variety of parameters
are fiber optic sensors. Such devices are used to monitor amongst
other parameters, temperature, pressure, stress, etc., in
machinery, power generators, manufacturing processes, propulsion
systems, and other applications.
PRIOR ART
[0005] U.S. Pat. No. 6,137,565, issued on Oct. 24, 2000 to Wolfgang
Ecke et al. discloses a temperature measuring system employing a
plurality of Bragg gratings responsive to temperature changes by
returning light signals from the sensors, the peak of which
indicates the value of the temperature being measured. The sensors
are coupled via optical fibers to an interferometer or a
spectrometer, with a linear photo detector, which converts these
signals into the temperature readings. The optical system, because
of its complexity, is expensive and, unlike the system described
herein, requires the use of pre-selected fibers, a significant
disadvantage. The use of a 2-D photo detector array in our
invention and by providing for interchangeability of all sensors
much simplifies the system and reduces its manufacturing cost.
[0006] U.S. Pat. No. 6,462,329, issued on Oct. 8, 2002 to Michael
A. Davis et al describes a sensor of ambient temperature utilizing
a Bragg grating used as a precision reference in testing thermistor
temperature sensors. There are significant differences between this
device and the subject of the present invention: Unlike the device
claimed herein, the device per U.S. Pat. No. 6,462,329 is not
designed for contact measurements of temperature. The Bragg grating
sensor is encapsulated, rather than exposed directly to the source
of heat. The heat to the Bragg grating is conducted from the
ambient environment via metal leads to the sensor, while we claim
the use of a Bragg stack on the distal end of a single fiber for
direct sensing of temperature.
[0007] U.S. Pat. No. 6,659,640, issued on Dec. 9, 2003 to Anthony
A. Ruffa, discloses a temperature measuring system that employs at
least two Bragg gratings as sensors that have a particular
coefficient of expansion vs. temperature responsive to a particular
wavelength, which means that the sensors have to be pre-selected
for specifically such parameter values. This procedure makes the
system more expensive as compared to the present invention in which
the temperature sensors only need to be within the spectral range
needed to accommodate the expected variation of the temperature or
other parameters of interest.
[0008] U.S. Pat. No. 6,788,835, issued on Sep. 7, 2004 to Besad
Moslehi et al. describes a complex opto-electronic system that
converts light signals returned from a pair of selected Bragg
gratings into the corresponding values of temperature being
measured. As in the case of U.S. Pat. No. 6,659,640, we believe
that this approach is more expensive and unnecessarily complex.
[0009] U.S. Pat. No. 7,084,974, issued on Aug. 1, 2006 to Andrej
Barwitz et al., claims a method for enhancing spectral information
first using a low resolution diffraction grating followed by
digital processing of the signals. The sensors are photoelectric
rather than Bragg stacks as in our invention. The system is
designed for analytical applications using a spectrometer, rather
than for measurement of temperature, pressure and stress.
[0010] U.S. Pat. No. 7,126,755, issued to John A. Moon et al on
Oct. 24, 2006 describes a spectrometer on a chip. Unlike the
present invention, it uses a linear, rather than two-dimensional
photo detector array to image the segments of the spectrum to
predetermined cells on the array. Each cell in the array is
assigned a numerical value, thus when a number of cells are
illuminated by the segments of the spectrum generated by the
diffraction grating, a unique code can be derived. Rather than
directly indicate the measured parameter magnitude, the code is
used to identify, sort, track, or verify a variety of materials
based on that code.
[0011] U.S. Pat. No. 7,130,041, issued to Ahmed Bouzid et al on
Oct. 31, 2006 for a method to implement the multispectral imaging
function with increased speed, simplicity and performance. A
two-dimensional photo detector array is used to receive a spectrum
of light dispersed by a grating such that each segment of that
spectrum is imaged on to a predetermined cell of the array. The
electrical signals emanating from the illuminated cells in the
array are combined to measure the optical energy within the
bandwidth of the spectrum. Our invention does not combine the
signals from the photo detector array, but rather uses the outputs
to identify from which of a plurality of sensors the signal
originates and what is the peak wavelength of such signal which
corresponds to the magnitude of the parameter of interest.
Furthermore, in the present system the initial sensing is done with
fiber optic sensors that have Bragg stacks deposited on their
distal ends, rather than photo electric detectors.
[0012] U.S. Pat. No. 5,118,200 issued to Dimitry M. Kiriov et al,
on Jun. 2, 1992 claims a system for remote measurement of
temperature in a process chamber. The measurement is implemented by
measuring the band gap of the heated substrate of interest by
comparing it with the continuous spectrum light from a light
source. In our system temperature is measured by determining the
shift of peak energy wavelength as a function of temperature or
other parameters of interest.
[0013] U.S. Pat. No. 6,895,132 issued to Behzad Moslehi et al. for
a system to measure strain using fiber optic sensors. In the
claimed system the peak wave length is determined using symmetrical
optical coupled fiber optic discriminators. Two channels each
accept the spectral signals from sensors employing Bragg gratings.
The optical energy in the two channels is compared. The result is
related to the strain. In an alternate embodiment, tunable filters
are used for wavelength discrimination. Our system does not employ
discriminators of the type described in this patent, nor does it
compare the outputs of the Bragg sensors.
[0014] U.S. Pat. No. 6,646,265 issued to Dale Marius Brown et al,
describes a special spectrometer designed to measure the
temperature of combustion flames. Two photo electric detectors are
employed to compare the optical energy of different parts of the
spectrum generated by the flame. In our system optical fibers with
Bragg stack deposited on their distal ends function as detectors.
We also claim a multiplexed network of Bragg sensors to allow
parameter measurements in different parts of objects. The outputs
of the sensors in the present invention are not compared, rather
the shift of the peak wavelength related to the magnitude of the
parameter being measured.
[0015] U.S. Pat. No. 6,698,920 issued to Donald Herbert Maylotte
for a system designed to measure the temperature of the buckets in
gas turbines. A plurality of detectors is employed that input their
signals into a spectrometer. An optical switch is used to select
the outputs of detectors placed in the line of sight. In our system
also a plurality of sensors is used which are distributed to
different location on the object to measure the parameters of
interest. We specified that sensors in our system will be optical
fibers with deposited Bragg stacks at their distal ends, such
sensors responsive to changes of temperature, pressure or stress
being monitored.
[0016] The Bragg stacks are structures of very thin layers of
diffractive ceramic materials, each layer having a different
refractive index, which function as very selective optical filters
or wavelength selective retro reflectors that are capable of
isolating the energy peak in a light spectrum and the position with
respect to wavelength of that peak. A practical method to use fiber
optics equipped with Bragg stacks is to transmit through the fiber
excitation light of sufficient bandwidth and intensity to the Bragg
stack at the distal end of the fiber and collect the reflected
light returned through the same fiber. The reflected light is
modified by the Bragg stack in that the energy peak is isolated in
the spectrum, the location of the peak within the reflected
spectrum being determined by the geometry of the Bragg stack.
[0017] One application of such sensors is in the measurement of
temperature. The Bragg stack at the end of the optical fiber is
brought in contact or in close vicinity to the object the
temperature of which is of interest. In response to the heat the
geometry of the Bragg stack changes in that the individual
refracting layers expand or contract depending on the temperature.
As a result the position of the energy peak is shifted with respect
to wavelength. Each position correlates with a particular
temperature, the value of which can be derived from an electronic
look-up table. An example of one embodiment of such system is
described in detail in WO 2005/017477 in International Patent
Classification.sup.1. .sup.1 Theodor Morse and Fei Luo, assigned to
Boston University
[0018] The same type of sensors can be used to measure pressure and
stress. In these applications the Bragg sensors at the end of
optical fibers are exposed to physical forces resulting from
pressure or stress in the objects being monitored, forces that
change the geometry of the Bragg stack. If high accuracy of such
measurements is important, separate sensor can be used to measure
the temperature of the object and utilize that information to
compensate the primary results.
[0019] Fiber optic sensors have many advantages over other types of
sensors for measuring the same parameters: [0020] Fiber optic
sensors are lightweight, flexible, have a very small diameter, can
be monitored from distances as long as several hundred meters, are
rugged, take up little space and are easily installed. [0021] Are
relatively easy to multiplex using a single multiplexing
spectrometer as readout. [0022] Work reliably in harsh and/or high
gravity environments. [0023] Generate no electro-magnetic
interference and are not affected by EMI [0024] Provide very fast
response. [0025] Are more cost effective, especially in multiplexed
networks [0026] When such sensors are used to monitor performance
of engines, the information so obtained may be used to ensure
optimal engine performance, save fuel, extend useful life, and
prevent catastrophic failures.
[0027] The fiber optic sensors, which may be used in the system of
this invention, utilize Bragg stacks as the actual sensing elements
at the distal ends of or within the fibers. The sensors of this
type are particularly effective in networks that allow
substantially simultaneous measurements and monitoring of
parameters of interest in several locations.
SUMMARY OF THE INVENTION
[0028] Briefly, in accordance with the present invention, a
multiplexing spectrometer is provided by photonic sensors, and in
particular with fiber optic sensors that incorporate Bragg stacks.
These sensors are multiplexed to provide the spectrometer
input.
[0029] A multiplexing spectrometer, in accordance with this
invention, includes a dispersive and imaging means used to
sequentially and rapidly interrogate fiber optic sensors and to
direct the optical signals from a plurality of such sensors to a
two-dimensional photo detector array. The signals from each of the
fiber optic sensors in the network are directed, using a
diffraction grating as a preferred embodiment, to a different cell
row on the two-dimensional photo detector array, each such row
having a fixed and known location. The fiber optic sensors have
deposited on their distal ends Bragg stacks elements, which change
their physical configuration in response to the changes in
temperature, pressure or stress in the devices they are monitoring.
A light source generates light of sufficient bandwidth to ensure
adequate spectral range to accommodate the shifting of the peak
response in the generated spectra due to magnitude-related changes
of the parameters being measured. The light source supplies the
input excitation for all of the fiber optic sensors by dividing the
light into a corresponding number of beams, which are directed
through optical fibers into the individual sensors. The Bragg
stacks function as variable filters that reflect the light back
into the fibers. The reflected light has an energy peak, the
spectral position of which depends on the current physical
configuration of the Bragg stack, responsive to the magnitude of
the parameter being monitored. The location of the peak varies as
the magnitude of that parameter varies.
[0030] At the input to the spectrometer the individual fibers are
fused into a vertical array; it is understood, however, that other
orientations of the fused fiber array are possible. The output from
this fiber array is projected within the spectrometer on to a
two-dimensional multi-cell photo detector array, such that the
light from each fiber within the fiber array is projected onto one
of predetermined cell rows within the photo array, which provide
horizontal space on the photo array for the spectral signatures of
the information coming from the sensors. Thus, the read-out
spectrometer determines the position and the magnitude of the
energy peak within the reflected light spectrum, converts the light
signals into corresponding electrical signals, and displays the
corresponding instantaneous magnitude of a parameter of interest as
well as the location of the specific sensor generating the
information. Features of the multiplexing spectrometer, in
accordance with the inventors, are the lack of moving components,
fewer components, lower manufacturing cost, and a compact package
that lends itself to miniaturization. Other important feature is
that all fiber optic sensors with Bragg stacks can be substantially
identical, i.e., respond to the changes of parameters by shifting
the position of the peak wavelength by the same amount and thus
ensure that the light signals are imaged on the same locations on
the matrix detector located in the multiplexing spectrometer. Also,
the same sensors, in addition to temperature, can be used for
pressure and stress measurements,
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 in a block diagram illustrates the excitation light
source and a method for distribution of light to the individual
fiber optic sensors.
[0032] FIG. 2 in a schematic diagram depicts the optical and
detection system of the multiplexing spectrometer.
[0033] FIG. 3 in a diagram, which shows schematically the
geometrical relation of the light output from the individual
sensors with the cells of the two-dimensional photo detector array
shown in FIGS. 1 & 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] Referring now to FIG. 1, which depicts as a block diagram
the excitation light source (01) preferably being a solid state
device such as a light emitting diode (LED) or super luminous light
emitting diode (SLED). Compared to more conventional light sources
such as incandescent lamps, the LEDs and SLEDs have the advantages
of much longer life, sufficiently wide bandwidth of the emitted
light, require much less power, operate of a low voltage power
supply, and allow very efficient coupling into the optical fibers.
In addition, because of their small size, such devices facilitate
miniaturization. The light source is connected to a power supply,
of the type familiar to those skilled in the art that compensates
for undesirable fluctuations in the intensity of light of these
light sources.
[0035] The optical module (02) serves to divide the incoming light
into as many channels as there are Bragg stack sensors; the block
diagram shows six sensors, but it is understood that the number of
sensors that can be used in the network, is limited only by the
magnitude of the light output of the light source and by the input
configuration of the spectrometer. From the module (02) the light
is directed through the fiber array (03A), through a set 50/50 beam
splitters (03C) and through the fiber array (03B) to the Bragg
sensors (05).
[0036] The light signals returned from the Bragg sensors (05)
through the fibers (03B) arrive at the beam splitters (03C) and are
directed by the beam splitters (3C) through the array of optical
fibers (04) to the optical module (06), which is shown in more
detail in FIG. 2, The individual fibers (04) bringing the light
signals from the sensors (05), are fused to form a vertical array
(13). The image of this array (13) is projected by mirror (11) on a
diffraction grating (12). The resulting spectrally and spatially
dispersed signal is imaged by the mirror (10) onto the
two-dimensional photo sensor array, which converts the light
signals to the corresponding electrical signals. These data are
then input into the electronics of the multiplexing spectrometer
(07), which includes an alphanumeric display. Module (08) stores
the data on defined locations on the photo detector (09) that
correspond specifically to the individual Bragg stack sensors
(05).
[0037] FIG. 2 illustrates a typical monochromator familiar to those
skilled in the art, the purpose of which is to isolate the peak of
light energy within the spectrum and to direct the light to the
two-dimensional matrix photo detector array (09). Such array can
comprise one of the many types of solid state detectors available
today, for example, Charge Coupled Devices (CCD), Charge Injection
Devices (CID), Complimentary Metal Oxide Semiconductors (CMOS),
Avalanche Diodes, and others. The choice of a particular type is
determined by the spectral response of the photo detector (09), as
well as its sensitivity, resolution, size, and cost. One of the
major criteria for the choice is the best response match for the
spectral output of the light source (01). The monochromator
configuration shown in FIG. 2 is known as Czerny-Turner, but it is
understood that other optical configurations could be used to
achieve the same result. The optical fibers (04) are fused into a
vertical array at the input to the multiplexed spectrometer (07)
optically coupled to the monochromator depicted in FIG. 2. From
there the light signals are imaged by the concave mirror (11) onto
the diffraction grating (12).
[0038] The spectrum from each sensor (05), FIG. 1, occupies a
different pre-selected position on the entrance slit (13). The
diffraction grating (12) separates the light signals being imaged
onto it by the concave mirror (11) and images the input slit on the
surface of the two-dimensional photo detector array (09). The
signals from each sensor (05) occupy one predetermined row on the
photo detector array. The photo detector (09) is being constantly
and rapidly scanned under control of the multiplexing spectrometer
(07). The light signals are converted by the photo detector (09)
into electrical signals. The multiplexing spectrometer (07) detects
the spectral peaks of each of the electrical signals and their
location on the photo detector array (09). Given this information,
the multiplexing spectrometer uses a built-in look-up table (08) to
determine which sensor (05) generates that light signal and its
magnitude and converts that information into the corresponding
values of the parameters being measured, such as temperature,
pressure or stress.
[0039] FIG. 3 further illustrates the relation of the Bragg stack
sensors to the photo detector array. In this drawing all
intermediary optical components have been omitted for the sake of
clarity. The light signals from the fiber optic sensor (2) are
directed to cell (Ab) in the matrix detector, meaning that
magnitude of the parameter being measured at that location is high,
corresponding to a shorter wavelength. The light signals from the
fiber optic sensor (6) are directed to the cell (Ff) indicating
lower energy and longer wavelength.
* * * * *